A conventional developing device incorporated in an electrophotographic image forming apparatus includes a developing roller and a supply roller. The developing roller supplies a developer to the photosensitive body. The supply roller supplies the developer to the developing roller. The developer moves in accordance with a voltage between supply roller and developing roller. Therefore, in order to move the developer from the supply roller to the developing roller, a predetermined voltage is set between the developing roller and the supply roller corresponding to type of the apparatus and a developer used in the apparatus.
If the image forming apparatus has been used for a long time, the image density may decrease with time. In view of this, the apparatus disclosed in Japanese unexamined patent application publication No. HEI 07-005765 changes a voltage between the developing roller and the supply roller in accordance with a remaining amount of the developer.
The image density decreases not only due to temporal change, but also due to a characteristic of image to be printed. If the image has a high-density, such as a black image, the developer needs to move in large amount from the developing roller to the photosensitive body. In some cases, the developer may not be supplied from the supply roller to the developing roller in a sufficient amount, inevitably lowering the image density. Even if image data is a specific pattern having low density, the uneven density may be conspicuous in the specific pattern. This uneven density may be insufficiently recognized as uneven printing when the developer is supplied insufficiently.
If the developer used is a non-magnetic two-component developer, strongly-charged component is more consumed than weakly-charged component, as the voltage is kept applied at a high voltage. This phenomenon is so-called “selective development.” Consequently, a ratio of the weakly-charged component to the strongly-charged component will increase, as the developer is consumed. Finally, images can no longer be developed even if the bias applied is increased.